The pulpal response to and cariogenicity of a third strain of Streptococcus mutans, namely S. mutans (NCTC 10832), was studied in monoinfected gnotobiotic rats of the Fischer strain using the techniques described previously by the present authors. Unlike S. mutans (NCTC 10449 and 10919), S. mutans (NCTC 10832) was associated with the presence of inflammatory cell infiltrates in the coronal pulp of a small number of teeth and extensive periapical inflammation 28 days after the creation of untreated pulpal exposures. S. mutans (NCTC 10832) was associated with the presence of extensive pulpal necrosis and reduced dentine bridge formation. These changes were similar to those noted with the other two strains of S. mutans. S. mutans (NCTC 10832) was non-cariogenic in monoinfected gnotobiotic rats of the Fischer strain.